Replace the dummy app with  `rails-dummy`.

See `spec/template.rb` on how the app is bootstrapped.

Remove cached spec/dummy.
Update the path to the cassettes to relative to our spec directory.
Cleanup spec helper and config files.
Cleanup the engine generators config.
Updated gems.

Use unreleased `easy_auth` and `easy_auth-oauth2` from Github.
Use features directory name instead of old requests.